Role of Secondary Sensory Cortices in Emotional Memory Storage and Retrieval in Rats

1. SCIENCE VOL 329 6 AUGUST 2010 Role of Secondary Sensory Corticesin Emotional Memory Storageand Retrieval in Rats Department of Neuroscience, University of Turin, Turin, Italy. National Institute of Neuroscience, Turin, Italy. Tiziana Sacco1 and Benedetto Sacchetti 1,2* 2010-10-11 Xin-jian Li

2. Main research interests • Role of the hippocampus (dorsal and ventral pole), amygdala (basolateral and central nuclei), cerebellum and neocortex (parietal, frontal and temporal area) in emotional and spatial learning • Neural analysis of spontaneous and learned behaviour: light/dark box paradigm, elevated plus maze task, passive avoidance response, fear conditioning, Morris water maze, place avoidance response, taste aversion learning • Cellular mechanisms of learning and memory: role of long-term potentiation (LTP) and long-term depression (LTD) • Molecular analysis of aversive memories: protein kinases, excitatory ionotropic and metabotropic glutamate receptors and emotional learning

3. Background During an emotional experience, sensory stimuli such as odors, sounds, and colors are associated with the affective qualities of that situation. How and where the brain stores permanent emotional memories remains elusive. Because memories involve the representation of past sensory and emotional events, they may be stored, in part, within the sensory cortex. Nonetheless, lesions of sensory cortices do not prevent the formation of emotional memories

4. The lesions have been performed before or shortly after learning, a time interval in which thalamus-amygdala circuits support the functional absence of sensory cortices. No data are available on the involvement of the sensory cortex in long-term storage and retrieval of emotional memories.

5. Materials and Methods Subjects: Male Wistar rats were used. Excitotoxic lesions were made in seventy-day old animals Neuronal cell loss was induced by injecting N-methyl-D-aspartic acid (NMDA) bilaterally. Remote memory Rats were trained to associate seven acoustic stimuli (i.e., conditioned stimuli, CSs) to an aversive unconditioned stimulus (US). The primary auditory cortex was lesioned 1 month later; then test the animal’s memory. Recent memory They underwent an CS-US training and 2 days later were tested for CS retention.

6. Schematic re resentation of the secondary sensory cortices included in the present study. The upper scale indicates positive and negative distances from bregma. Amg, amygdala; Ent, entorhinal cortex; Oc1, primary visual cortex; Oc2L and M, secondary occipital visual cortex; Pir, piriform cortex; Te3, tertiary auditory cortex.

7. Fig. 1. Primary auditory cortex and fear memories. (A) Histological reconstruction of the smallest (gray) and largest (hatched) excitotoxic lesions aimed at the primary auditory cortex, area Te1. Negative numbers indicate posterior distance from bregma. Ent, entorhinal cortex; Gu, gustatory cortex; Par, parietal cortex; PRh, perirhinal cortex; Te2 and Te3, secondary and tertiary auditory cortex. (B) Remote fear memories to acoustic CSs in Te1-lesioned (n = 11) and sham-operated (n = 10) animals.

8. Fig. 2. Secondary auditory cortex and emotional memories. (D) Recent fear memories inTe2-lesioned (n = 12) and sham-operated (n = 13) animals. (A) Histological reconstruction of the excitotoxic lesions aimed at the Te2 area. (B) Schematic representation of the secondary sensory cortices included in the present study. (C) Remote fear memories in Te2-lesioned posterior perirhinal–lesionedand sham-operated animals.

9. Lesions of the secondary auditory cortex, but not of the posterior perirhinal cortex or primary cortex, affect long-term fear memory. Post-acquisition lesion of the auditory cortex does not abolish recent fear memories.

10. Fig. 3. Lesions of secondary visual cortex or of posterior piriform cortex impair remote fear memories. Extent of Oc2L lesions. (B) Remote fear memories to visual CSs in lesioned and sham-operated animals and in those retrained. (C) Recent emotional memories in and sham-operated rats.

11. Fig. 3. Lesions of secondary visual cortex or of posterior piriform cortex impair remote fear memories. (D) Extent of posterior piriform cortical lesions. (E) Animals with posterior piriform cortex lesions are impaired in retention of remote memories relative to sham animals . Lesioned animals were able to relearn CS-US Association (F) Posterior piriform cortex lesions do not affect recent fear memories. Perirhinal cortex (PRh) extent from. AIP, agranular posterior insular cortex; Ent, entorhinal cortex; Gu, gustatory cortex; Oc1, primary visual cortex; Te2 and Te3, secondary and tertiary auditory cortex.

12. Lesions of the secondary cortices affect remote fear memories, whereas recent memories are left intact. These results suggest that amnesia, when present, is specifically related to memory impairment and is not secondary to sensory or motor disturbance.

13. To more fully address this issue, They used the cell permeable zeta inhibitory peptide (ZIP). ZIP inactivates protein kinaseMz (PKMz), an autonomously active protein kinase C isoform that is required for encoding long-term memory traces in several brain site. We injected ZIP into Te2, Oc2L, or piriform cortex 1 day or 1 month after fear learning. Memory was tested 2 days after injection.

14. Fig. 4. Impairment of remote fear memories after injection of the PKM inhibitor ZIP. (A) ZIP or the scrambled version of ZIP was injected into Te2 1 day after training. (B) ZIP injection into Te2 1month after training hampered remote memory relative to control animals. (C) ZIP-treated rats did not show memory spontaneous recovery group × trial interaction, (D) Recent memory to visual CSs was similar in rats infused with ZIP or scr-ZIP (E and F) ZIP administration into Oc2L affected remote memories relative to control animals tested 2 (E) and 2 weeks (F) after injection. (G) Recent olfactory fear memories were similar in ZIP treated and control Rats ZIP inactivates protein kinase (H and I) ZIP injection into posterior piriformCortex impairedremote memory both 2 days (H) and 2 weeks (I) afterinjection ZIP: Cell permeable zeta inhibitory peptide

15. whether each cortex encodes memories specifically related to the sensory modality elaborated by the area, or whether these sites play a general role in memory processes irrespective of the diverse sensory modalities present during the emotional experience.We therefore investigated the effect of lesions of each sensory cortex on fear memories related to CSs of different sensory modalities.

16. Fig. 5. Role of sensory cortices in emotional memory. (B) Visual fear memory in control, Te2-lesioned, or posterior piriform–lesioned animals. (A) Acoustic fear memories in control ,Oc2L lesioned, and posterior piriform cortex–lesioned rats. (C) Olfactory fear memory in control.Te2-lesioned ,or Oc2L-lesioned rats.

17. Two possibilities: Secondary sensory cortices may encode the physical features of the sensory stimuli (“recognition memory”). Alternatively, these sites may store the emotional meaning acquired by sensory stimuli during an emotional experience. whether lesions of secondary cortices impair long-term recognition memory of stimuli not associated with any overt emotional consequences. Sudden auditory stimuli elicit a startle response; if the stimuli are presented repeatedly, startle response habituates across days and this habituation is retained for weeks. Long term habituation memory relies on the memory of the sensory stimuli presented repeatedly.

18. Fig. 5. Role of sensory cortices in emotional memory. (E) Remote fear memories to white-noise CSs were impaired in Te2-lesioned rats relative to sham-operated animals.Te2-lesioned animals could form new fear memories. (D) Startle abituation during the first day of training, during the retention trial, and during the presentation of a new tone in Te2-Lesioned and sham-operated rats and in those untrained. (F) Olfactory habituation in the posterior piriform–lesioned animals and in shamoperated animals.

19. An alternative method to investigate the involvement of neural sites in memory processes is based on the analysis of expression of early genes such as cfos and zif268. Such genes are required for synaptic plasticity and are used as an index of neuronal activation. We tracked the level of the proteins encoded by zif268 in Te2 to investigate (i) whether this site is recruited by recent and/or remote fear memories, (ii) whether it is engaged by emotional and/or sensory memory, and (iii) the cortical layer(s) activated.

20. Fig. 6. zif268 protein expression in sensory neocortex and in the amygdala. (A and B) Photomicrographs of zif268 staining within Te2 cortical layers I–VIafter testing for remote (A) or recent (B) fearful memory. Scale bar, 150 mm. (C) After remote memory test, zif268 counts relative to naïve animals significantly increased in conditioned rats in layers II–III and IV. (D) After recent fear memory test, zif268 analysis revealed no difference among groups in Te2 cortical layers II–III, IV, V, and VI. (E to G) After remote memory test, zif268 was also analyzed in the lateral, basal, and central nuclei of the amygdala. Scale bars, 230 mm. (H) Lateral (LA) and central (Ce), but not basal (B),amygdala activity was significantly enhanced after remote memory retrieval mainly in the tone-conditioned group. All data are means T SEM

21. Excitotoxiclesions of auditory, visual, or olfactory secondary sensory cortices impaired remote, but not recent, fear memories in rats. Amnesia was modality-specificand not due to an interference with sensory or emotional processes. In these sites, memory persistence was dependent on ongoing protein kinaseMz activity and was associated with an increased activity of layers II–IV, thus suggesting a synaptic strengthening of corticocortical connections. Lesions of the same areas left intact the memory of sensory stimuli not associated with any emotional charge. We propose that secondary sensory cortices support memory storage and retrieval of sensory stimuli that have acquired a behavioral salience with the experience.